Dr Seyed Safavi would like to make it clear that he was conducting research on his own behalf, and not for his brother General Yahya Rahim Safavi, as we stated in the article below.

Iran dramatically raised the stakes in its tense diplomatic stand-off with Britain last night, broadcasting a propaganda video of the British sailors and marines seized last week, including a "confession" that they had entered Iranian waters.

The Foreign Office reacted furiously to the video, calling it "completely unacceptable" and expressed "grave concerns" about the conditions under which Leading seaman Faye Turney was persuaded to admit on film that the 15-strong British naval patrol had strayed into Iranian territory last Friday.

Britain has started moves towards a UN security council resolution condemning the seizure of the personnel and the TV screening. The defence secretary, Des Browne, said the refusal to release the sailors was unacceptable. Britain had not planned to go to the UN until next week when it takes over the security council chairmanship, but last night South African ambassador Dumisani Kumalo, the current council president, said the UK had circulated a press statement on the hostages to the 14 other council members. He said the statement would be discussed today.

The video was shown by al-Alam, an Iranian satellite channel broadcasting across the Middle East in Arabic. Leading seaman Turney was shown wearing a headscarf and makeup, and smoking while giving an account of the incident, which was translated and voiced over in the broadcast. "Obviously we trespassed into their waters," she is shown saying against a floral backdrop. "They were very friendly and very hospitable, very thoughtful, nice people. They explained to us why we've been arrested; there was no harm, no aggression."

The video was not shown in Farsi to a domestic Iranian audience. It included footage of other marines and sailors sitting and eating in a nondescript room, showing no obvious signs of injury. It also showed a handwritten letter purporting to be by leading seaman Turney to her parents, saying she had "written a letter to the Iranian people to apologise for us entering into their waters".

"I wish we hadn't because then I'd be home with you all right now," the letter said.

Downing Street believes the admissions were made under duress, and is not convinced that the screening is a precursor to the sailors' release. Number 10 said they would continue to ratchet up the pressure slowly. "The next few days will be [used] to increase Iran's sense of diplomatic isolation," a government official said.

The Iranian embassy in London said it had handed the letter to the British government, adding that the captive crew were "in good health and condition and they enjoy welfare and Iranian hospitality".

"We understand the anxiety of their families, but they must be assured that they are in safe hands and have a better life than the risky mission in the Persian Gulf waters," the Iranian statement said.

The foreign secretary, Margaret Beckett, condemned the video and the release of the letter, saying she was particularly disappointed that a private letter has been used in a way which could only add to the distress of the families.

Last night Iran's foreign minister said his government had agreed to allow British officials to meet the hostages, although he did not specify when the visit could occur. Manouchehr Mottaki told the Associated Press that Britain must admit that its sailors entered Iranian waters for the standoff to be resolved. If the crew's alleged entry into Iranian waters was a mistake "this can be solved. But they have to show that it was a mistake, that will help us to end this issue."

Earlier, Mr Mottaki had suggested that Leading seaman Turney, the only woman among the 15 captives, would be released "as soon as possible", although this is hardly likely to defuse a crisis which appeared to escalate by the hour yesterday. The broadcast of the video came soon after a British announcement that it was cutting off official contacts with Tehran on any business apart from the naval detainees.

The Ministry of Defence also issued a detailed account of the seizure of the naval patrol last Friday, with charts, map coordinates and photographs supporting Britain's insistence that it was well within Iraqi waters when it was surrounded by Iranian gunboats. Mrs Beckett also alleged that the Iranian government had changed its story over the past few days in an attempt to support its contention that the two British patrol boats had entered Iranian waters near the Shatt al-Arab waterway separating Iraq from Iran.

Britain had already begun canvassing its partners in the UN security council and the EU, seeking solidarity in the showdown with Iran. Ministers are pinning their hopes on Turkey and Germany as the main two levers on Tehran.

At an Arab League summit in Riyadh, the UN secretary general, Ban ki-Moon, Turkish prime minister Recep Tayyip Erdogan and Iraqi foreign minister Hoshyar Zebari all raised the fate of the captives with Mr Mottaki at Britain's behest.

A Foreign Office spokesman said of the video broadcast last night: "Given the nature of Leading seaman Faye Turney's statement and the apparent confession that the personnel were 'arrested after they trespassed into Iranian waters' we have grave concerns about the circumstances under which she made this statement."

Patrick Cronin, director of studies at the International Institute of Strategic Studies, said the crisis represented a counterattack by Tehran radicals after months of international pressure over Iran's nuclear programme. "They clearly want to change the subject. They want to go on the offensive."

There is evidence that the 15 sailors and marines were captured and are being held by the Islamic Revolutionary Guard Corps, (IRGC) which represents a state within a state, with its own forces, its own political representatives and its own hardline ideology. Al-Alam is thought to have ties to ultra-conservative factions in Tehran.

Iran is seeking the release of five officials arrested by US forces in Iraq in January, who the Americans claim are senior members of the IRGC. However, the Iranian foreign ministry has denied Tehran is seeking a prisoner swap.

Last Tuesday, the brother of the IRGC commander, General Yahya Rahim Safavi, was in London and said he was conducting research on behalf of his brother. He appeared to be particularly interested in the threat of further economic sanctions by Europe, according to sources with knowledge of the meetings.

This letter was released yesterday by Iran, which claims it was written by Faye Turney to her parents. It contains some oddly-worded phrases and there are concerns it was written under duress:

Dear mum and dad

I am writing to you from Iran where I am being held. I will try to explain to you the best what has happened.

We were out in the boats when we were arrested by Iranian forces as we had apparently gone into Iranian waters. I wish we hadn't because then I'd be home with you all right now. I am so sorry we did, because I know we wouldn't be here now if we hadn't.

I want you all to know that I am well and safe. I am being well looked after. I am fed 3 meals a day and have a constant supply of fluids.

The people are friendly and hospitable, very compassionate and warm. I have written a letter to the Iranian people to apologise for us entering into their waters.

Please don't worry about me, I am staying strong. Hopefully it won't be long until I am home to get ready for Molly's birthday party with a present from Iranian people.